Digital Elevation Model (DEM) is the terminology adopted by the USGS to describe terrain elevation data sets in a digital raster form. The standard DEM consists of a regular array of elevations cast on a designated coordinate projection system. The DEM data are stored as a series of profiles in which the spacing of the elevations along and between each profile is in regular whole number intervals. The normal orientation of data is by columns and rows. Each column contains a series of elevations ordered from south to north with the order of the columns from west to east. The DEM is formatted as one ASCII header record (A-record), followed by a series of profile records (B-records) each of which include a short B-record header followed by a series of ASCII integer elevations per each profile. The last physical record of the DEM is an accuracy record (C-record). 1-degree DEM (3- by 3-arc second data spacing). Provides coverage in 1- by 1-degree blocks. Two products (three in some regions of Alaska) provide the same coverage as a standard USGS 1- by 2-degree quadrangle. The basic elevation model is produced by or for the Defense Mapping Agency (DMA), but is distributed by the USGS in the DEM data record format. Coverage is for the Delaware River Basin.
DEM's can be used as source data for digital orthophotos, and, as layers in geographic information systems, for earth science analysis. DEM's can also serve as tools for volumetric analysis, for site location of towers, or for drainage basin delineation.
1-degree DEM's have rows and columns which are based on the geographic coordinate system. The use of this system results in a rectangular DEM which shares a common edge and therefore duplicate points with other adjacent 1-degree DEM's.
ground condition
None Acknowledgement of the U.S. Geological Survey would be appreciated in products derived from these data.
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The accuracy of a DEM is dependent upon the level of detail of the source and the grid spacing used to sample that source. The primary limiting factor for the level of detail of the source is the scale of the source materials. The proper selection of grid spacing determines the level of content that may be extracted from a given source during digitization.
The fidelity of the relationships encoded in the data structure of the DEM are automatically verified using a USGS software program upon completion of the data production cycle. The test verifies full compliance to the DEM specification
The DEM is visually inspected for completeness on a DEM view and edit system for the purpose of performing a final quality control and if necessary edit of the DEM. The physical format of each digital elevation model is validated for content completeness and logical consistency during production quality control and prior to archiving in the National Digital Cartographic Data Base. Level 2 DEM: Level 2 DEM's may contain void areas due to interruptions to contours in the source graphic or DLG. Void area elevation grid posts are assigned the value of -32,767. In addition, suspect elevation areas may exist in the DEM but are not specifically identified. Suspect areas can be located on the source graphic as a "disturbed surface" which are symbolized by contours overprinted with photorevised or other surface patterns.
Digital elevation models meet horizontal National Map Accuracy Standards (NMAS) accuracy requirements. NMAS horizontal accuracy requires that at least 90 percent of points tested are within 0.02 inches of the true position. The primary reference from which 1-degree DEM accuracy is derived is source-based, as a minimum, meeting NMAS for 100,000-scale USGS topographic series maps.
Digital elevation models meet horizontal National Map Accuracy Standards (NMAS) accuracy requirements.
The vertical root-mean-square error (RMSE) statistic is used to describe the vertical accuracy of a DEM, encompassing both random and systematic errors introduced during production of the data. The RMSE is encoded in element number 5 of record C of the DEM. Accuracy is computed by a comparison of linear interpolated elevations in the DEM with corresponding known elevations. Test points are well distributed, representative of the terrain, and have true elevations with accuracies well within the DEM accuracy criteria. Acceptable test points include, in order of preference: field control, aerotriangulated test points, spot elevations, or points on contours from existing source maps with appropriate contour interval. A minimum of 28 test points per DEM is required to compute the RMSE, which is composed of a single test using 20 interior points and 8 edge points. Edge points are those which are located along, at, or near the quadrangle neatlines and are deemed by the editor to be useful to evaluating the accuracy of the edge of the DEM. Collection of test point data and comparison of the DEM with the quadrangle hypsography are conducted by the quality control units within the USGS. There are three types of DEM vertical error; blunder, systematic and random. These errors are reduced in magnitude by editing but cannot be completely eliminated and may be encountered. Blunder errors are those errors of major proportions and are easily identified and removed during interactive editing. Systematic errors are those errors that follow some fixed pattern and are introduced by data collection procedures and systems. These error artifacts include: vertical elevation shifts, misinterpretation of terrain surface due to trees, buildings and shadows, and fictitious ridges, tops, benches or striations. Random errors result from unknown or accidental causes. DEMs are edited to correctly depict elevation surfaces that correspond to water bodies of specified size. Level 2 DEM: A vertical RMSE of one-half of the contour interval (of 100,000-scale source), determined by the source map, is the maximum permitted. Systematic errors may not exceed one contour interval, determined by the source map, is the maximum permitted. Systematic errors may not exceed one contour interval specified by the source graphic. Level 2 DEM's have been processed or smoothed for consistency and edited to remove identifiable systematic errors.
Hypsographic vector information which is interpolated to regular grid posts to form DEM grids in 3- by 3-arc second data spacing within the 1-degree DEM bounds.
Elevation data in the form of a digital elevation model regridded to 3- by 3-arc second data spacing within the 1-degree DEM bounds.
DEM's are created by gridding digital line graph (DLG) contours or the equivalent from USGS 1:24,000-scale or 1:100,000-scale maps. DEM's may also be created from regridding 7.5-minute DEMs to the coarser 3-arc second grid spacing of the 1-degree DEM. The accuracy and data spacing are intended to support computer applications that analyze hypsographic features to a level of detail similar to manual interpolation of information from printed source maps. Quality control: DEM's are viewed on interactive editing systems to identify and correct blunder and systematic errors. DEM's are verified for physical format and logical consistency at the production centers and before archiving in the National Digital Cartographic Data Base (NDCDB) utilizing the Digital Elevation Model Verification System (DVS) software. Additional information on this software may be found in Department of the Interior, U.S. Geological Survey, 1993, Technical Instructions, DVS Users Manual (draft): Reston, VA
The Binghamton E&W, Scranton E&W, Newark E&W, Wilmington E&W, Harrisburg E, & Salisbury E 1:250,000 DEMS were downloaded. They were appened together and cut into a window that contains the Delawae River Basin and reprojected to Albers.
The digital elevation model is composed of a 6-character integer raster representing a gridded form of a topographic map hypsography overlay. Each raster entity contains a 6-character integer value between -32,767 and 32,768.
U.S. Department of the Interior, U.S. Geological Survey, 1992, Standards for digital elevation models: Reston, VA A hypertext version is available at: <URL: ftp://www-nmd.usgs.gov/pub/dem_html/standards_dem.html> A hypertext extract is available at: <URL: http://edcwww.cr.usgs.gov/glis/hyper/guide/1_dgr_dem> Softcopy in ASCII format is available at: <URL: ftp://www-nmd.usgs.gov/pub/ti/DEM/demstnds/stdempt1.txt> <URL: ftp://www-nmd.usgs.gov/pub/ti/DEM/demstnds/stdempt2.txt> <URL: ftp://www-nmd.usgs.gov/pub/ti/DEM/demstnds/stdempt3.txt> Softcopy in WordPerfect format is available at: <URL: ftp://www-nmd.usgs.gov/pub/ti/DEM/demstnds/stdempt1.wp5> <URL: ftp://www-nmd.usgs.gov/pub/ti/DEM/demstnds/stdempt2.wp5> <URL: ftp://www-nmd.usgs.gov/pub/ti/DEM/demstnds/stdempt3.wp5> Softcopy in PostScript format is available at: <URL: ftp://www-nmd.usgs.gov/pub/ti/DEM/demstnds/stdempt1.ps> <URL: ftp://www-nmd.usgs.gov/pub/ti/DEM/demstnds/stdempt2.ps> <URL: ftp://www-nmd.usgs.gov/pub/ti/DEM/demstnds/stdempt3.ps>
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Although these data have been used by the USDA Forest Service, no warrenty expressed or implied is made by the USDA Forest Servie as to the accuracy of the data. The act of distribution shall not consitute any such warranty, and no responsibility is assumed by the USDA Forest Service in the use of these data, software, or related materials
It is a standard Arc Info inport/export file.
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